Secretary of State John Kerry and national intelligence advisers arrive on Capitol Hill on Friday to update members of the House on Syria's alleged use of poisonous gas in its ongoing civil war. US intelligence has concluded with 'varying degrees of confidence' that the Syrian government has twice used chemical weapons in its civil war, the White House and other top administration officials said Thursday.

“We’ll be ready to present them when we’re asked,” adds the senior US military official.

Mr. Hagel said Thursday that “any use of chemical weapons in Syria very likely originated” with the Assad regime, adding that the use of the “uncontrollable, deadly weapons” in turn “violates every convention of warfare.”

Some Washington lawmakers, in turn, called for an armed US response. “I think it’s pretty obvious that a red line has been crossed,” Sen. John McCain (R) of Arizona said Thursday. “Now I hope the administration will consider what we have been recommending now for over two years of this bloodletting and massacre – and that is to provide a safe area for the opposition to operate, to establish a no-fly zone, and provide weapons to the people in the resistance who we trust.”

Others urged caution. “Based on the intelligence that we have, it appears that the Assad regime has used chemical weapons against the Syrian people,” said Rep. Adam Smith of Washington, the highest-ranking Democrat on the House Armed Services Committee. Even so, he added, “I am not convinced that military action is appropriate at this time. There is no evidence military action will achieve anything, except cost American lives and treasure.”

US military officials note that arming rebels in Syria remains a risky proposition, because the allegiances of those doing the fighting are unclear.

“If we are going to give them something more sophisticated, is it going to get into the wrong hands?” asks a senior Pentagon official.

To that end, military officials are seeking “clarity of who we’re dealing with and also clarity of outcome,” General Dempsey said Friday.

To that latter point, Pentagon officials caution that it’s not clear that providing arms to the rebel forces will shift the tide of the war.

“The main thing is, will it make a difference?” says the Pentagon official. “This country is awash in weapons.”

Night vision goggles, body armor, and communications gear to help rebel factions coordinate with one another might be even more effective tools, they say.

For now, US military officials say they continue to plan – and await political directive.

“Nothing I’ve learned over the last 24 hours changed what I’ve been doing,” Dempsey said Friday.

That said, “With the outcome identified,” he adds, “I can produce a military option to achieve it.”